GLOBAL LAND PRECIPITATION INCREASES IN 20TH CENTURY

Global land precipitation has increased during the 20th
century, especially at the mid and high latitudes, according to a
paper published in the November 1997 issue of the Journal of Climate.

The paper, written by scientists Drs. Inez Fung, Anthony Del
Genio, and Aiguo Dai, is based on a recalibrated compilation and
analysis of data from 1900-1988 and confirms previous speculation
that land precipitation is increasing. The new research shows a
global land trend of a 2.4 mm per decade increase in annual
precipitation amounts. Multiplied by almost nine decades, this
means that there is about 22 mm more rain falling now each year
than there was at the turn of the century -- rainfall as a global
mean has risen by slightly more than two percent.

"Though much speculation remains as to the cause of this
increase, further long-term study is needed to help ascertain the
reasons for this change. The research does show, however, that
both the spatial pattern and rate of precipitation increase are
reminiscent of global climate model predictions of the
atmosphere's response to an increase in greenhouse gas
concentrations," said Dr. Anthony Del Genio, research scientist at
NASA's Goddard Institute for Space Studies (GISS), New York City.

NASA scientists learned of this rise in precipitation from a
new data set constructed at GISS. Scientists analyzed the data at
face value but, in the process, used mathematical techniques to
detect patterns of historical errors. Researchers developed new
ways to objectively determine variations and remove ones that are
not accurate. Finally, they performed a statistical test to
determine the numerical confidence and came up with the revised
database, which they believe shows the long term changes more
precisely than previous analyses of the data.

"This database represents a potentially valuable resource for
understanding the nature of land precipitation variations and
their role in climate processes," said Dai, a researcher at GISS,
a branch of NASA's Goddard Space Flight Center, Greenbelt, MD.

For example, the research analysis confirms the global
patterns of the presence of an El Nino and also depicts the well-
known Sahelian drought of the past few decades that has been a
major influence over parts of Africa.

Through the construction of this historical database,
researchers confirmed the occurrences of 24 droughts and five
floods world-wide in the 20th century, though most took place in
the tropics rather than middle latitudes.

Scientists have long held that precipitation is one of the
most important aspects in Earth's climate system because of its
impact on the global biosphere. In addition, precipitation limits
the amount of water vapor in the atmosphere. Water vapor is the
most important greenhouse gas and thus helps to determine Earth's
surface temperature. The amount of water vapor in the atmosphere
determines when and where clouds form.

"The latent heat released when water precipitates out of the
atmosphere is the primary way in which the tropical ocean
'communicates' with the atmosphere and drives the tropical
atmospheric circulation," said Del Genio.

Tracking precipitation and its relationship to global
climate, however, has been difficult because the data have not
been recorded into coordinated databases until recently. In
addition, precipitation measurements vary widely across small
geographic areas making it difficult to measure accurately.
Through further study, NASA and researchers at GISS hope to create
more accurate models from which conclusions about global rainfall
can be drawn.

NASA also is due to launch this month the Tropical Rainfall
Measuring Mission (TRMM), the first mission dedicated to measuring
tropical and subtropical rainfall through microwave and visible
infrared sensors. The TRMM mission includes the first spaceborne
rain radar. Tropical rainfall comprises more than two-thirds of
global rainfall. Data from the TRMM mission should greatly
enhance researchers' understanding and prediction abilities of
global climate change.

The ongoing research at GISS is funded through NASA's Mission
to Planet Earth Enterprise, a long-term coordinated research
effort to study the total Earth system and the effects of natural
and human-induced changes on the global environment.